This invention relates to bearing arrangements in the form of cylindrically tubular bushes or cylindrically part tubular shells mounted against radial expansion and in particular relates to such arrangements in which the bush or shell comprises a plastically deformable and ductile lining material comprising or including a low friction polymer-based material and fillers, referred to hereinafter as a filled polymer, that is compressible in the sense of being less than fully dense before compression. Such a filled polymer based bearing material is, in the following description and claims, referred to as xe2x80x9cfilled polymer compressible liningxe2x80x9d.
The invention is particularly, but not exclusively, concerned with such bearing arrangements in which the filled polymer compressible lining is defined by a filled polymer infiltrated into a sponge-like sintered metal matrix to form the lining material, which matrix may itself be carried on a backing strip of solid metal, such as steel or bronze or like material commonly used as a backing material in bearing applications. It is known to form bearing arrangements in which the sintered metal matrix is bronze and such lining material is herein referred to as xe2x80x98filled polymer infiltrated sintered bronzexe2x80x99 or xe2x80x98FPISBxe2x80x99 for short, and as xe2x80x98backed filled polymer infiltrated sintered bronzexe2x80x99 or xe2x80x98BFPISBxe2x80x99 for short, respectively.
Such BFPISB is manufactured and sold by the Applicant with various low friction polymer materials and filled with different combinations of fillers. For example, the material known as DU has a polymer base of PTFE with lead filler and is described in patent specification no GB-A-2172296, the material known as DP which has a polymer base of PTFE and zinc and fine bronze instead of lead and is described in patent specifications no GB-A-2248238, the material known as DP4 which has PTFE with fillers of calcium fluoride and fibrillated Kevlar (RTM) and is described in patent specification no GB-A-2279998, and the material known as DU (B) which has the same lining material and bronze sinter as DU but a bronze backing instead of steel. The above is not an exhaustive list, and bearing bushes and shells of such bearing material are employed, for example, in vehicle door hinges to support a rotatable hinge pin and in suspension components to support reciprocating rods, both in rotational and rectilinear motion.
A typical steel backed, filled polymer infiltrated bronze bearing material is formed as a laminar strip manufactured by depositing bronze powder onto a steel backing, heating the combination to sinter point to develop a sponge-like bronze matrix layer, depositing a solvent-borne paste or mush of the low friction polymer and fillers, rolling to infiltrate it within the interstices of the bronze layer and again heating the combination to dry and sinter the filled polymer so that it completely impregnates the bronze sinter matrix. It is a feature of such manufacture that the filled polymer not only infiltrates the interstitial spaces between the partially fused bronze particles (leaving only about 1% porosity) but also forms a relatively thin skin overlying them. Depending upon the eventual use of the bearing arrangement an amount of filled polymer may be used that causes such skin to exist in a thickness of 0.010 mm to 0.04 mm. Such structure is sometimes described in terms of the polymer skin being an overlay that is intimately bonded to a substrate provided by the fused metal particles, the substrate being itself intimately bonded to a backing strip where appropriate.
In a typical manufacture, tubular bush bearings are formed by deforming or bending a laminar plate-like blank, cut from such strip, around a cylindrical mandrel into a longitudinally slit tubular form having the ISB lining innermost. The tubular form, which is of course circumferentially discontinuous and unstable against radial forces, is thereafter mounted within a radially stronger housing for reception of hinge pin or like cylindrical object to be borne thereby.
It will be appreciated that in the manufacture of the tubular bush form there will be variations in dimensions, particularly the inside and outside diameters thereof as defined by the mandrel diameter and strip thickness, and that furthermore the variations will be exacerbated when the tubular form is finally mounted within a separately manufactured housing that is itself subject to manufacturing tolerances.
Whereas a bush having a conventional bearing metal lining or homogeneous (incompressible) polymer can be manufactured to have an undersized inside diameter and have lining surface material removed by a reaming tool or the like to achieve a desired nominal inside diameter, it is not readily possible to provide inside diameter accuracy for, in particular, an ISB bearing by removing lining material.
To more clearly illustrate the steps involved in production of such a known bush bearing arrangement having an ISB lining and understand constraints placed upon achieving dimensional accuracy, reference is made to FIGS. 1(a) to 1(f), the bush and its manufacture being known in the art and briefly described here as an aid to understanding the invention.
Referring to FIG. 1(d), a bearing arrangement 10 is defined by a tubular bush form 12 that has a circumferential discontinuity 14 and is mounted within a radially constraining housing 16. Referring also to FIG. 1(a) a laminar strip 18 of steel backed ISB bearing stock, having steel backing 19 and ISB lining 20, manufactured as outlined above and with a width corresponding substantially to the desired bush length, is pulled from a coil 22 and at a trimming station 24 its edges are trimmed by chamfering to prepare the eventual bush ends 121 and 122. The strip is fed to a blanking station 26 at which a predetermined length is a cropped to form a plate or blank 30. The cropped blank is positioned with the ISB surface 20 adjacent a cylindrical mandrel 32 that has a circular cross section of predefined diameter. Referring also to FIG. 1(b), a first ram 34 clamps the blank to the mandrel and bends the blank about the mandrel into a U-shape; thereafter a pair of second rams 361 and 362 close it around the mandrel into substantially tubular form before a third ram 38 applies pressure to the ends of the strip, the rams in unison pressing the blank against the mandrel to effect tubular uniformity with the mandrel so that the erstwhile opposite ends of the blank meet as a circumferential discontinuity of the tubular form. Referring also to FIG. 1(c), the tubular form, indicated generally art 40, therefore has its tubular wall, conveniently identified as 42, corresponding substantially to the thickness of the BISB 18. The tubular form 40, still held closed on the mandrel is displaced with the mandrel through a die 44 which defines or gauges the outside diameter of the tubular form and, relative to the mandrel, the thickness of the tubular wall 42. Any changes in wall thickness necessary to permit it to pass through the die are the result of elongation or drawing of the radially confined components of the wall which retain their relative thicknesses, although there may be a certain amount of recovery of wall thickness as the materials leave the die. The mandrel is thereafter withdrawn from the tubular form to leave the bush.
The structure of the wall on an enlarged scale is illustrated in FIG. 1(e), illustrating not only backing strip 18 and ISB 20 but also within the ISB the sintered bronze matrix 201, filled polymer 202 and the polymer surface layer 203.
As a consequence of the circumferential discontinuity 14, the bush 40 is relatively weak against radial forces and it is mounted for use within an encircling housing 16 to comprise the aforementioned bearing arrangement 10.
As discussed above, manufacturing tolerances in respect of BISB strip thickness and the operations associated with forming of the bush on the mandrel and gauging or defining wall thickness prior to removing it from the mandrel give a distribution of inside diameter values and, to a lesser extent, wall thickness (and thus outside diameter) values each within a range, conveniently called herein a variation range. By way of example, in manufacturing a bush of the order of 20.00 mm inside diameter from bearing stock 18 of some 1.5 mm overall thickness (comprising steel backing 19 of 1.2 mm thickness and ISB layer 20 of some 0.3 mm thickness), the internal diameter of the tubular form may be distributed within an industrially reproducible manufacture in a variation range of 0.04 mm or thereabouts, that is, having a maximum inside diameter IDMAX greater than minimum inside diameter IDMIN by a variation IDVAR=IDMAXxe2x88x92IDMIN=approx 0.04 mm, and a variation range of wall thickness of 0.01-0.02 mm.
This degree of dimensional accuracy in inside diameter of the bush per se may be considered marginally acceptable in respect of receiving an elongate member to be borne by the bush, but it is found that as a result of mounting such bush within the separately manufactured housing, which may itself have a distribution of inside diameters, the effective IDVAR for the assembled bush may be greater than 0.08 mm.
As also discussed above, the ISB bearing surface precludes reaming of an undersized bush to the desired inside diameter by removal of excess lining material. However, the ISB material is amenable to so-called burnishing by running through the mounted bush a hard, smooth body of diameter greater than the maximum inside diameter of the mounted bush, which body applies radial pressure to the wall by way of the lining surface and, in a manner analogous to the above-described gauging of the outside diameter of mandrel-borne bush by a surrounding die, effects a reduction in overall wall thickness by drawing or causing the wall components to flow lengthways of the bush within the radially inexpansible housing, similarly maintaining their relative thicknesses.
Referring to FIG. 1(f), a fragment of the arrangement of FIG. 1(d) is shown schematically on an enlarged scale along with a fragment of, a burnishing tool 50 which has a body formed from a rod of tool steel hardened and tempered to 58-62 ARC, the rod comprising a slight bi-conical taper (shown greatly exaggerated) of about 1xc2x0 at end regions separated by a cylindrical central portion 51 having a length less than the bush, typically 20% of bush length. In respect of burnishing such a smooth bore bush the burnishing tool has a cylindrical diameter that exceeds IDMAX by about 30-50% of IDVAR.
By way of example, bearing arrangements were produced from a sample number of bushes formed as described above which exhibited inside diameters varying between 20.00 and 20.05 mm, that is, with an IDVAR of about 0.050 mm. The burnishing tool 50 had a nominal cylindrical diameter of 20.072 mm (in the range 20.070 to 20.075 mm that represents the tolerance of tool diameter), that is, exceeding IDMAX by about 45% of IDVAR. The burnishing tool was pushed through the mounted bush in the direction of arrow 54 and, as illustrated in a schematic way, with the steel backing 19 supported by the housing 16, the cylindrical portion of the tool applied local radial pressure to the BISB 20 which is compressed between the tool and lining.
Apart from any marginal degree of porosity of the lining material 20, it is essentially incompressible and behaves homogeneously so that passage of the tool requires a thinning of the wall 42 as a whole by flow of all of the components axially; such thinning retains the relative thicknesses of the component layers of the wall and comprises a mixture of plastic deformation wherein the bush is elongated permanently and elastic deformation whereby the wall regains some of its thickness after the tool has passed, principally due to the backing being deformed well within its elastic limits of the steel. The inside diameter after burnishing is a function of the extent to which the diameter of the burnishing tool exceeds the inside diameter of the unburnished bush and the recovery (which is itself a function of the degree of compression that is related to the excess of the total diameter), but in this example, and as seen from the Figure, the after-burnishing inside diameter distribution between IDBMIN and IDBMAX (=IDBVAR) was less than before burnishing and typically to the level of variation achieved for the bush manufacture per se eliminating the additional variation due to mounting in the housing; typically, for IDVAR=0.050 mm, IDBVAR=0.025 mm.
However, there are a number of difficulties and limitations attached to implementing such burnishing technique. Because of the nature of the lining material it is only possible to effect an increase in the inside diameter of such a mounted bush by reducing the overall thickness of the wall which, as is seen from the above description is manifested as an elongation of the bush by xe2x80x98flowxe2x80x99 of the components of the wall in response to the radial compression exerted by the burnishing tool. However, the deformation of the bush wall, which comprises largely the steel backing strip, is governed by the behaviour of the steel which, for a relatively small extension envisioned in this situation, exhibits a non-negligible, and unpredictable, degree of recovery. That is, the diameter of the burnishing tool has to be sufficiently large in relation to the initial inside diameter that it causes greater deformation of the bush wall than is really required in order to allow for this recovery. This greater deformation not only requires an energy input which is larger than the final change suggests but also that level of initial deformation/energy input is limited by the level at which the accompanying longitudinal forces begin to shear the polymer material from the lining. Thus, in respect of applying such burnishing to known mounted bushes of constant wall thickness, not only is a considerable amount of energy wall deformation required to push the burnishing tool through the bush but that the recovery in wall thickness thereafter represents a continued uncertainty in respect of final inside diameter and together they limit the extent to which the wall can be deformed to effect a specific increase in inside diameter.
Furthermore, the radial pressure resulting from such driving force and compression may, in some cases, be too great for the strength of the housing which may cease to give support to the bush. As the mounting of the bush may have to be effected at its point of use rather than in circumstances given over to bush manufacture, such burnishing of the mounted bush may therefore also have to be effected near the point of eventual use, where conditions are not conducive to such difficulties as may accompany use of the burnishing tool.
Although the above discussion has concentrated upon tubular bushes and bearing arrangements in which the tubular bush form is retained when mounted, that is, wherein the bearing is circularly sectioned and substantially fully cylindrical, it will be appreciated that such mounted bush bearings may be divided longitudinally into discrete arcuate shells which are mounted for radial support and used singly or as a pair, that is, a circularly sectioned, but only part-cylindrical bearing. The inside diameter dimension of a shell or pair of shells may be established whilst in tubular bush form before division or, for a pair of shells, after mounting with respect to each other into a bush-like tubular form, so that the technique of burnishing a tubular form mounted in a radially constraining housing may be considered applicable to shell-type ISB bearings as for tubular bush ISB bearings.
Also the structures and techniques are applicable to lining materials comprising the various filled polymers that are compressible and suitable for such lining use and/or without a solid metal backing.
Such lining structures and burnishing method provide a starting point for the present invention, and it is an object of the present invention to provide a method of manufacturing a circularly sectioned bearing having a burnished, radially supported filled polymer compressible lining with improved dimensional tolerance and ease of manufacture than hitherto, and a circularly sectioned, burnished tubular bush bearing arrangement and associated burnishing tool for such manufacture.
It is also an object of the present invention to provide a circularly sectioned bush form, having a filled polymer compressible lining, suitable for mounting surrounded by a radially constraining housing and then having its inside diameter defined by a burnishing tool when mounted, that permits easier passage of a burnishing tool and exhibits greater dimensional accuracy resulting from such passage. It is furthermore an object of the present invention to provide a substantially laminar bearing material having a lining filled polymer compressible and suitable for bending to form such a circumferentially discontinuous bush.
According to a first embodiment of the present invention a method of making a circumferentially sectioned bearing having a burnished, radially supported filled polymer compressible lining comprises (i) defining a tubular form, of which the filled polymer compressible lining presents a bearing surface extending about, and facing inwardly towards, a longitudinal axis, having an internal diameter smaller than the desired internal diameter of the bearing, (ii) mounting the tubular form in a radially restraining housing to define a mounted form, and (iii) increasing the internal diameter of the mounted form to said desired internal diameter by passing therethrough a burnishing tool, having a cylindrical portion of length less than the length of the tubular form and diameter in excess of the desired internal diameter of the bearing and operable to effect by said passage compression of the filled polymer compressible lining in a direction substantially perpendicular to the surface, and is characterised by the step of, prior to passage of a said burnishing tool through the mounted bush form, effecting at least a partial compression of a minor part of the filled polymer compressible lining as a plurality of depressions in said bearing surface of the mounted tubular form, distributed over the surface.
According to a second embodiment of the present invention a bearing bush arrangement comprises (i) a circularly sectioned tubular bush form surrounding a longitudinal axis and manufactured with an outside diameter dimensional to locate within a radially outwardly constraining housing and an internal diameter less than that desired of the bearing bush arrangement said manufactured internal diameter being defined by a bearing surface of filled polymer compressible lining facing radially inwardly towards said longitudinal axis, and (ii) an associated burnishing tool, adapted to be passed through the housed tubular form along said longitudinal axis and having a cylindrical portion of diameter in excess of the desired internal diameter of the bearing bush arrangement, the arrangement being characterised in that said tubular bush form is manufactured to have, prior to passage of the burnishing tool therethough, said lining partially compressed over a minor part of the bearing surface as a plurality of depressions distributed over the surface.
According to a third embodiment of the present invention a bearing material comprises a substantially laminar, bendable strip having a bearing surface defined by a filled polymer compressible lining and wherein a minor part of the lining is partially compressed as a plurality of depressions distributed over the surface.